Prairie

by Candace Savage

  Eocene herbivores

  At Toadstool Geologic Park, near Chadron, Nebraska, sediments that originally eroded off the young Rocky Mountains were subsequently thrust up by faulting and then sculpted by erosion. The park also features a trackway of fossil footprints left millions of years ago by giant pigs, rhinos, camels, and other prehistoric beasts.

  Life was easy. But then a sequence of unrelated events halfway around the world sent the climate into a nosedive. (According to one theory, the separation of Antarctica from Australia caused a major rerouting of oceanic currents, with the result that water from the poles no longer mingled with water from the equator. The South Pole thus became an isolated refrigeration cell that eventually spread a chill around the entire planet.) Beginning about 37 million years ago, the average global temperature dropped by 14˚F (8˚C) over the over the span of a million years. Thereafter, despite brief periods of recovery, the climate continued to cool. As the weather became cooler and drier, the tropical forests of the North American plains began to wither and die away.

  But conditions that were death for palm trees were ideal for another group of plants. Relative newcomers on the evolutionary scene, grasses had first appeared shortly after the extinction of the dinosaurs but had met with limited success. They were drought specialists, and while humid conditions prevailed, they had been confined to small patches of ground that had somehow been deprived of abundant rainfall. Now, not only were the tropical rains failing because of a global drying trend, but the North American plains were under a special disadvantage. With the Rockies in place, storms that rolled in from the Pacific tended to drop their precipitation as they swept up the western slopes. By the time they reached the plains, they were pretty much wrung out. But grasses don’t require much moisture, and this characteristic gave them a competitive edge. Over the next several million years (between about 24 million and 3 million years ago), grasses gradually became the dominant plants across the Great Plains.

  If we could slip through a crack in time and go back to the plains of Nebraska some 20 million years ago, we would find ourselves in a landscape that is at once familiar and wonderfully strange. This is big-sky country, an open landscape of shoulder-high grasses dotted with walnuts and other broad-leafed trees, vaguely reminiscent of the savannas of East Africa today. A broad river courses across the plain, its margins fringed by willows and its current murky with sediment from the constantly eroding Rockies. Whenever this river floods, it coats the land with yet another layer of silt and sand.

  The river is the main source of water in this increasingly arid land, and wildlife flocks to its banks. Herds of miniature rhinos (about the size of domestic pigs but with two horns sprouting from the ends of their snouts) plunge into the shallows to find refuge from biting flies. Ancestral horses called Parahippus, somewhat bigger than Orohippus but still the size of dogs, come down to the river to drink at dawn and dusk. The rest of the time, they range across the savanna, plucking leaves off the trees and grazing on grasses that tower over their heads. Because grass is very abrasive, Parahippus have acquired specially ridged teeth that are able to withstand the daily grinding. Llamalike camels (members of a family that evolved in North America and only later migrated to South America and Eurasia) lounge in the willows but keep an eye out for any suspicious shadows moving through the bushes. In this world, danger takes the forms of saber-toothed cats and long-jawed dogs, some of them as large as coyotes and wolves. Smaller dogs, the size of foxes, prey on the Paleolagus, or “ancient rabbits,” that burrow into the roots of shade trees, and on Paleocastor, or “ancient beavers,” that, amazing as it seems, occupy deep, corkscrew burrows in the middle of the dry prairie.

  One of the distinctive species of the northern and central Great Plains, the white-tailed jackrabbit traces its ancestry back to the Miocene Period, some 37 million years ago.

  Dwarf rhinoceros

  Orohippus and Parahippus against the silhouette of the modern horse, Equus

  Paleocastor

  Traces of these animals, and others like them, have been preserved at the Agate Fossil Beds National Monument on the Niobrara River in northwestern Nebraska. Here, the buried beds of bone testify not only to remarkable lives but also to miserable deaths. It seems that the drying trend, which had driven back the rain forest and allowed the lush parklands to spread, occasionally became so severe that it stressed even the savannas, causing rivers to dry up and trees to blacken. Animals gathered alongside the dying rivers and died along with them. Later, when floods flashed down out of the mountains, the currents gathered up the bones, massing them into backwaters and oxbows.

  As the centuries ticked by, the climate became progressively more arid. Soon, in place of the lush savannas, a tawny, almost-treeless grassland sprawled across the plains. And although many mammalian species survived—including rhinos, horses, camels, rodents, cats, and dogs—all were challenged by their changed and unforgiving environment. An unremitting diet of grass pushed grazing animals to develop high-crowned teeth, which grew in to replace themselves as they were worn away. The absence of hiding places put a premium on speed, forcing both predator and prey to adopt the runner’s long-legged physique. Hunter and hunted also came to rely on their quick wits, as the brain power of both players was augmented.

  As it turned out, these hard-won adaptations would offer little protection against the trauma that was about to unfold—the Ice Age.

  * * *

  > PRAIRIE MOUNTAINS

  The Black Hills, which straddle the border between Wyoming and South Dakota, are the most easterly outliers of the Rocky Mountains. (On some geological maps, they are actually identified as the Central Rocky Mountains.) These hills rose up out of the plains between 62 million and 48 million years ago. The core of the hills, extending roughly from Deadwood to Wind Cave, is a massive, elliptical dome of granite, some of it as much 2.5 billion years old, that was thrust up from deep underground and exposed by erosion. Now carved into spires and peaks, the dome lies shoulder deep in a broad, encircling apron of younger rocks, formerly ocean floors, that is known as the Limestone Plateau.

  At one time, about 37 million years ago, this plateau was completely buried in sediments that had washed down off the dome, but that overburden— and much more besides—has since been washed away. Erosion has also created the remarkable Racetrack, or Red Valley, that runs around the foot of the plateau. But nothing has been able to wear away the hogback of resistant Cretaceous sandstone that encloses both valley and hills like a fortress wall. Chosen by Americans as a site to honor past presidents (four of whom are represented on Mt. Rushmore), the Black Hills also stand as a natural monument to the colossal energies that shaped the continent.

  * * *

  Permanent Winter

  Nobody knows for sure why the cold settled in as it did. Perhaps (as one theory suggests) the chill from the Antarctic refrigeration cell crept gradually north until the Arctic Ocean froze and exerted its own cooling force. Or maybe the expansion of the continental land mass over several hundred million years had caused the global temperature to trend downward. (Since land holds less heat than water, a larger land mass might logically translate into lower temperatures.) The rise of the Rockies and other mountain ranges around the world may also have contributed to the decline by disrupting the jet stream and causing cold Arctic air to spill south across the land. Or perhaps all these Earthbound events were irrelevant in the grandeur of space, where a wobble in the Earth’s orbit and the inconstancy of the sun may have triggered subtle changes in the climatic system.

  Be that as it may, between about 3 million and 2 million years ago, the Earth had cooled so much that permanent winter had settled over the northern reaches of the continent. The tepid summers no longer melted away the preceding winters’ snows. Beginning at high latitudes and progressing southward, drifts built up into mounds, and mounds into mountains, until the snow compacted into ice under its own tremendous mass. Eventually, after several thousand years, these
glaciers began to advance, flowing almost imperceptibly but relentlessly south over the Central Lowlands. In time, the northern third of North America was buried under some 2 miles (3 kilometers) of ice; that’s about the height, from base to peak, of Mount Everest. In its heartland on the Precambrian Shield, the ice reached a maximum depth of about 16,000 feet, or 5,000 meters.

  Geologists used to believe that the glaciers advanced and retreated four times over a span of about 2 million years. These successive incursions were known in North America as the Nebraskan, Kansan, Illinoian, and Wisconsin glaciations, in honor of their southernmost extent. But more recent research suggests that the glaciers probably made many more than four sweeps down the continent, each time grinding away the traces left by previous glaciations. Since much of the record has been wiped clear, a detailed chronology of the Ice Age on the prairies cannot be reconstructed. But we do know that by about 1.2 million years ago, a vast slab of ice had bulldozed its way almost to the present-day confluence of the Missouri and Mississippi rivers. At its maximum, the ice sheet probably extended beyond the Canadian provinces, sweeping across northeastern Montana and south through the Dakotas to northeastern Kansas. From there it cut across the plains of northern Missouri and then eastward, across the continent, to the ice-stricken valley of the St. Lawrence.

  Glacial erratics—boulders of granite from the Canadian Shield that were carried south by advancing ice—still stand where they were dropped at the end of the last glaciation.

  After that ice sheet (the Kansan) retreated, the glaciers never again penetrated quite so deeply into the plains. The final glaciation, for example, which began some 100,000 to 75,000 years ago, didn’t progress much farther south than central Iowa. But the devastation that the glaciers inflicted was not limited to their actual footprint. Whenever the glaciers melted back, they left behind outwash plains of sand and silt. Ferocious winds that developed over the ice fields picked up this grit and hurled it around the interior of the continent. In a number of places (notably, the Great Sand Hills of Saskatchewan and the Sand Hills of western Nebraska) the wind laid down its burden in vast fields of dunes. Elsewhere, the storms whipped up clouds of dust—rock that had been ground into flour by the glaciers—and broadcast it over the land. Today, these silt, or loess, deposits, often several yards thick, form the bluffs along the Iowa side of the Missouri River and provide the matrix for rich, rolling farmlands in Nebraska, Kansas, Iowa, Missouri, and elsewhere.

  The ice began its final, halting retreat about 18,000 years ago, a transition that marked the beginning of the present interglacial period. (Unless the buildup of greenhouse gases warms the climate enough to break the Ice Age cycle of retreat and advance, a new glaciation can be expected to begin within the next several thousand years: some experts expect it within decades.) Over the succeeding 10,000 years, a block of ice larger than present-day Antarctica gradually melted away, and it didn’t go quietly. Torrents gushed from the eroding ice sheets, gouging out meandering coulees and wide flat-bottomed river valleys as they coursed eastward over the plains. Today, dry coulees writhe incongruously across the northern prairies, from nowhere to nowhere, and glacial spillways seem ludicrously oversized for the quiet streams, like the Milk River, that now occupy their broad channels.

  And it wasn’t only moving water that left its mark on the land. In many places, meltwater was prevented from flowing away by ice dams, and the silt-laden water pooled to form shallow, milky lakes, such as Glacial Lake Regina in south-central Saskatchewan and Lake Dakota in east-central South Dakota. The largest of these “proglacial” lakes, Glacial Lake Agassiz, flooded some 135,000 square miles (350,000 square kilometers) at its maximum extent (three times the size of Lake Superior, the largest modern freshwater lake), including extensive tracts in Saskatchewan and Manitoba and the Red River Lowland in eastern North Dakota and northern Minnesota. When the ice and then the water finally retreated from the land, these lake bottoms stood exposed as wide flat plains bounded by terraced beaches, all covered with a dressing of mineral-rich silt.

  The land that emerged directly from under the ice sheets, by contrast, was a rough-and-tumble mess, strewn with the rubble that the glaciers had dropped as they retreated. Sinuous ridges of gravel and silt, called eskers, marked the courses of streams that had once flowed under or through the ice; strange conical hills called kames stood where streams pouring out of the glaciers had deposited gravel and sand. One of the most prominent glacial features on the northern plains was a long, broken ridge of hill country, called the Missouri Coteau, that meandered (and still meanders) across central Saskatchewan and south through the Dakotas. Geologists refer to the Coteau as “dead ice moraine,” because it formed when hunks of ice became buried in gravel and lay there for centuries, ever so gradually rotting away. As each block of ice melted, the gravel that had been lying on top of it sagged to form a depression, or prairie pothole.

  Meanwhile, south of the reach of the glaciers, on the foreshore of the Rockies, the landscape had also been undergoing renovations. Sometime before the Ice Age set in, the entire western prairies had inexplicably begun to rise. As a result, the rivers, which previously had been building up the plains with loads of gravel and silt, now began to cut through the very layers they had previously deposited. This erosion was most dramatic along the slopes of the Rock-ies, where the rivers were powerful enough to wear through 70 million years of sediments. Along the Front Range of the mountains in Colorado, for example, the South Platte and Arkansas rivers have dug down about 1,600 to 2,000 feet (500 to 600 meters) below the level of the High Plains, right down to—and into—the seabeds of the Cretaceous. Only where erosion-resistant layers of rock have stood against this assault can the remnants of the older landscape be seen. The tops of the buttes and mesas that tower over the eroded plains were once a part of this continuous high plain.

  Saber-toothed cat

  By the end of the glaciation, the Great Plains of North America had been transformed from the seabed of ancient times into a mosaic of distinctive landscapes. To the north extended a rumpled terrain of glacial debris. Beyond the limit of the glaciers, to the south and east, lay a softer landscape of ancient ocean floors, much of it now blanketed in wind-shaped drifts of glacial sand and silt. Across these rolling hills to the west, the flatlands of the High Plains stepped up steadily toward the front ranks of the Rockies. And everywhere, rivers were cutting down into the land, etching deep valleys, canyons and, where the land was suitably dry and bare, badlands.

  But if the varied landforms of the plains were beginning to look more like those of the present, many of the life-forms still did not. Disadvantaged by the cool, wet weather of the Ice Age, the grasses that had previously dominated the plains had lost ground to other plants. Now a band of tundra skirted the retreating ice, while to the south, dark coniferous forests spanned much of the continent. Pure grasslands were restricted to scattered meadows and, perhaps, to a relict prairie crammed into the southernmost plains. Together, these diverse habitats were occupied by a stunning array of life, including white-tailed and mule deer, caribou, several species of pronghorns, black bears, cougars, bobcats, lions, cheetahs, saber-toothed cats, horses, llamas, one-humped camels— even Ice Age elephants. Woolly mammoths (shaggy beasts that stood 10 feet, or almost 3 meters tall) browsed on the tundra, while Columbian mammoths (just as unkempt and much larger) appear to have favored the remnant patches of grassland. Meanwhile, in the forests, their somewhat daintier relatives, the mastodons (the size of Indian elephants) fed on a diet of black-spruce boughs and other woody tidbits.

  The mammoths and mastodons were relatively recent arrivals on the plains, Ice Age immigrants that migrated across the Bering land bridge from Eurasia during intermittent cold spells. Whenever the climate worsened and the glaciers advanced, water became locked up in the ice and sea levels dropped, exposing a bridge of land across the Bering Strait between Siberia and Alaska. When the glaciers receded again, the land bridge was drowned, but a passageway si
multaneously opened to the south through the Canadian plains, which allowed the newcomers to wander into the heart of the continent. Some mammals, including ancestral camels and horses, made this journey in reverse, moving north when the plains corridor was open and then migrating across to Asia when the land bridge appeared.

  Columbian mammoth

  Skulls of Bison latifrons and Bison antiquus

  Plains bison, Bison bison

  Of all the species that arrived on the North America plains during the Ice Age—a menagerie that included not only elephants but also grizzlies, elk, and moose—two demand special notice. The steppe bison, Bison priscus, was a magnificent, thick-maned animal with flamboyant curving horns (attributes that are dramatically depicted in the cave art at Lascaux, France). The first bison herds likely poured across the Bering land bridge a few hundred thousand years ago and eventually made their way south to the Great Plains. Over the millennia that followed, successive waves of steppe bison made the same long trek, eventually meeting and mingling with the descendants of the pioneer herds. Meanwhile, that pioneer stock had been changing, shaped by life on the steppes and forests of a new continent. The result of this complex process of immigration, adaptation, and interbreeding was the creation of several distinctively North American types, notably the giant, long-horned Bison latifrons and the somewhat smaller Bison antiquus. In time these species were displaced by an even more compact version, Bison bison, the shaggy beast that, in historic times, provided food and shelter to the first people of the plains.